Why is the center of ground beef sometimes brown in color?

In steaks and roasts packaged in overwrap film, the color of the outside of the cut is bright red. Within the cut, it is purple because there is no oxygen to cause the meat to "bloom" (term used in industry to signify the conversion from the purple state to the red state in the presence of oxygen).

In making ground beef, some air is introduced in the grinding process. When ground beef is packaged in overwrap film, plenty of oxygen is available to generate the bright red color of lean on the surface. Because there is some, but not too enough oxygen deep within the product, it causes

the meat to turn brown. When the product is allowed to come in contact with oxygen, it usually will bloom to the bright red color like the surface.